Battery separators are porous diaphragms placed between the positive and negative plates of a battery so that the electrolyte forms the sole internal conducting path between them. Generally, separators have ribs on the side presented to the positive plate and are flat on the side presented to the negative plate.
It is generally considered to be desirable in the battery separator art to employ the smallest possible pore size since this reduces the danger of active materials being forced through or growing through the separator thereby causing shorting between the plates thereof or other detrimental effects. Also, a separator should have a relatively low electrical resistance.
U.S. Pat. No. 3,351,495 discloses a battery separator having a relatively low pore size and satisfactory electrical resistance characteristics made from a high molecular weight polyolefin having an average molecular weight of at least 300,000, a standard load melt index of substantially zero, and a reduced viscosity of not less than 4. The separator is manufactured by extruding the high molecular weight polyolefin in admixture with an inert filler and a plasticizer by the subsequently extracting the plasticizer by the use of a suitable solvent.